Stiebel Eltron DHC-E 8/10 instant water heater

[PeterW]

10 minutes ago, joe90 said:

the two immersions are half way up the tank and slightly above that, so immersions will only heat at most half a tank. Unless the tank does  not stratify well??? Is this correct or not?

 

I changed the spec and had a single immersion moved to the bottom of the tank to make sure the E7 charge does the whole tank. 

[ProDave]

I expect the water in the tank will circulate well when it is heating up. I am pretty sure the heat pump coil goes virtually top to bottom in the tank.  It is more a case that once warm and shut off, the thermostat in it's pocket won't sense much of a temperature drop until nearly 1/3 of the hot water has been used up. So you could well start your showering with only just over 200L of hot water available.

 

My immersion is quite low down so I expect when the PV is powering that it will heat the lot well.

[Jeremy Harris]

1 minute ago, ProDave said:

I expect the water in the tank will circulate well when it is heating up. I am pretty sure the heat pump coil goes virtually top to bottom in the tank.  It is more a case that once warm and shut off, the thermostat in it's pocket won't sense much of a temperature drop until nearly 1/3 of the hot water has been used up. So you could well start your showering with only just over 200L of hot water available.

 

My immersion is quite low down so I expect when the PV is powering that it will heat the lot well.

 

Bit like us having showers thinking that the Sunamp UniQ is charged right up, when in reality it may only be half charged, because of the daft way the Qontroller works...

[ProDave]

3 minutes ago, PeterW said:

You’ll find that you’re not getting full temperature as the way the coils work - and the sheer size of the coil - then it needs a slightly higher temperature to get the cutoff.  

Mine heats the water until the themister in it's pocket senses the temperature you have asked for.  The flow temperature gets to about 55 to achieve that.

[PeterW]

@ProDave are you not using the stats provided with the tank ..?? What are you using as an overheat stat ..?

[ProDave]

3 minutes ago, PeterW said:

@ProDave are you not using the stats provided with the tank ..?? What are you using as an overheat stat ..?

The ASHP came with a thermistor to put into a pocket.  That is the primary temperature control.

 

In the other pocket is the mechanical dual tank thermostat supplied with the tank.  That is set to a higher temperature so never normally opens, but if something makes the tank overheat, that will open and remove power to the 2 port valve feeding the tank.

[Ed Davies]

Since one of the more credible failures is the 2-port valve sticking, wouldn't it be safer if the overheat thermostat cut off the pump?

 

But is there really a need for an overheat stat for an ASHP? Firstly, the heat pump will limit its own flow temperature to something reasonable. Then, if that fails too, by the time the tank gets up to nearly 100 °C won't the ASHP be operating so inefficiently, if at all, that it'll barely keep up with the losses from the tank. Actually, that would be an interesting experiment for next summer, see how hot the ASHP can get the tank.

[ProDave]

I pondered what to do with the mechanical thermostat. It is not needed for normal control as would be with a conventional boiler. The heat pump takes care of controlling the temperature with it's own thermistor. But it seemed wrong to do nothing with it.

 

The heat pump itself cannot possibly heat the water to anything like boiling, so poses no danger to the cylinder.  But this particular heat pump contains a willis type in line immersion heater as well. So a possible failure mode is the contactor that controls it, or the controls failing and that sticking on.  I don't know what safety precautions LG built in, but I would expect as a very minimum it would have the conventional immersion heater thermostat so even then overheating should not happen.

 

But in any event connecting the mechanical thermostat to shut the 2 port valve seemed a simple and obvious safety feature.

[Dave Jones]

interesting thread. I'm having the same issue with ASHP planning as we are a 5 person household and any limit on hot water is a no go.  I'm thinking of fitting a combi just for the showers (4 of them) to get around the lack of hot water.  Obviously could use electric showers but they are just not as good as combi/mains and would be a backward step which I wont take.

 

It's the heat pump achilles heal which I cant see any other way out off.

[SteamyTea]

1 hour ago, Dave Jones said:

I'm having the same issue with ASHP planning as we are a 5 person household and any limit on hot water is a no go.  I'm thinking of fitting a combi just for the showers (4 of them) to get around the lack of hot water.  Obviously could use electric showers but they are just not as good as combi/mains and would be a backward step which I wont take.

 

It's the heat pump achilles heal which I cant see any other way out off.

Just a matter of sizing correctly, have you thought of having two ASHPs?

[Dave Jones]

17 hours ago, SteamyTea said:

Just a matter of sizing correctly, have you thought of having two ASHPs?

 

would 2 provide limitless hot water for showers ?

[SteamyTea]

10 minutes ago, Dave Jones said:

would 2 provide limitless hot water for showers

No, or yes.

Only a very large ASHP could supply limitless DHW, as it would be running like a combi boiler.

My thinking for having two ASHPs is that you can split your DHW into two separate systems, think of it as two separate houses.

 

It is, as usual, a case of just running some numbers and seeing what comes out.

So the first thing is to decide how much water you need to heat, that sets the cylinder/s size, then how fast you want to heat them, which sets the ASHP size.

Edited November 30, 2020 by SteamyTea

[Dave Jones]

done all that. even looked at putting a 300L tank for each shower. Still crap compared to a combi though and LOT greater expense for a worse result.

 

It's just a compromise i cant live with. A bit like buying a ferrari and it wont start 1/50 times.

[PeterW]

55 minutes ago, Dave Jones said:

even looked at putting a 300L tank for each shower. Still crap compared to a combi though and LOT greater expense for a worse result.

A combi won’t do 4 showers - even a Greenstar 550i HiFlow will max out at 25l/min which is pushing it for even 3 showers as the main pipework in the boiler itself is only 22mm. 
 

Average shower is around 80 litres of DHW. For 4 showers, and assuming you want to go gas (as you mention combis) then go for a 400 litre UVC with a 4sqm coil, sometimes called a fast recovery coil, and put a 40Kw system boiler in and set the tank temp to 65c. It will keep up with the initial drop but a 40Kw system boiler can dump enough heat in to keep you above 48c which is the usual drop off point on lost shower mixers. Your bigger issue with this will be sheer flow rates assuming you can get a dynamic 3Bar at 50 litres a minute.

 

May even be worth looking at the higher pressure UVCs (6Bar) and running the DHW split into shower feeds and then reducing the other basin / sink feeds to 3Bar and installing flow restrictors too. 
 

If you wanted to go ASHP rather than gas then you’ve got to look at recovery time, and then consider stepping up to a 500 litre UVC with a 16KW or even 25kW 3 phase ASHP and potentially an inline heater to provide belt and braces. All perfectly doable but needs proper planning and you need to start with the flow requirement and work backward to see just how much hot water you need to store. 

[SteamyTea]

26 minutes ago, PeterW said:

need to start with the flow

Yes, both into the property and out of the taps.

[Dave Jones]

2 hours ago, PeterW said:

A combi won’t do 4 showers - even a Greenstar 550i HiFlow will max out at 25l/min which is pushing it for even 3 showers as the main pipework in the boiler itself is only 22mm. 
 

Average shower is around 80 litres of DHW. For 4 showers, and assuming you want to go gas (as you mention combis) then go for a 400 litre UVC with a 4sqm coil, sometimes called a fast recovery coil, and put a 40Kw system boiler in and set the tank temp to 65c. It will keep up with the initial drop but a 40Kw system boiler can dump enough heat in to keep you above 48c which is the usual drop off point on lost shower mixers. Your bigger issue with this will be sheer flow rates assuming you can get a dynamic 3Bar at 50 litres a minute.

 

May even be worth looking at the higher pressure UVCs (6Bar) and running the DHW split into shower feeds and then reducing the other basin / sink feeds to 3Bar and installing flow restrictors too. 
 

If you wanted to go ASHP rather than gas then you’ve got to look at recovery time, and then consider stepping up to a 500 litre UVC with a 16KW or even 25kW 3 phase ASHP and potentially an inline heater to provide belt and braces. All perfectly doable but needs proper planning and you need to start with the flow requirement and work backward to see just how much hot water you need to store. 

 

thanks peter food for thought.